// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics;
using System.Net.Mime;
using System.Text;

namespace System.Net.Mail
{
    // RFC 2822 Section 3.2.2 - Quoted Characters
    // As in C# strings, characters that would otherwise have special meaning should be ignored when quoted/escaped
    // by a backslash "\". Quoted characters (quoted-pair) are only allowed in the following contexts: Comments,
    // quoted-string, domain-literal, and message-id.
    //
    // Example: A quoted-string ("hello there") cannot contain a double quote, as it must be surrounded by them.
    //  However, the double quote can be included as a quoted-pair: ("hello\"there") means (hello"there).
    //
    // Because backslashes themselves can be quoted-pairs (\\), this class's primary function is to verify what
    // character is being quoted. In "hi\\\\a", 'a' is not quoted because all of the backslashes are paired together.
    // In "hi\\\a", 'a' is quoted.
    internal static class QuotedPairReader
    {
        //
        // Counts the number of consecutive quoted chars, including multiple preceding quoted backslashes
        //
        // Preconditions: Index should be within the bounds of the data string.
        //
        // Return value:
        // - 0 if the char at data[index] was not quoted
        //   e.g. (\\a) given index=2 returns 0 because 'a' is not quoted
        // - The number of consecutive quoted chars, including multiple preceding quoted backslashes
        //   e.g. (a\\\b) given index=4 returns 4, as 'b' is quoted, and so are the previous backslashes
        //
        // Throws a FormatException or false is returned if the an escaped Unicode character is found but was not permitted.
        internal static bool TryCountQuotedChars(string data, int index, bool permitUnicodeEscaping, out int outIndex, bool throwExceptionIfFail)
        {
            Debug.Assert(0 <= index && index < data.Length, $"Index out of range: {index}, {data.Length}");

            if (index <= 0 || data[index - 1] != MailBnfHelper.Backslash)
            {
                outIndex = 0;
                return true;
            }

            // Count the preceding backslashes
            int backslashCount = CountBackslashes(data, index - 1);

            // For an even number of backslashes, the original character was NOT escaped/quoted
            if (backslashCount % 2 == 0)
            {
                outIndex = 0; // No quoted pair to skip
                return true;
            }
            else
            {
                if (!permitUnicodeEscaping && !Ascii.IsValid(data[index]))
                {
                    if (throwExceptionIfFail)
                    {
                        // Cannot accept quoted Unicode
                        throw new FormatException(SR.Format(SR.MailHeaderFieldInvalidCharacter, data[index]));
                    }
                    else
                    {
                        outIndex = default;
                        return false;
                    }
                }
                // Skip the quoted char, and the odd number of backslashes preceding it
                outIndex = backslashCount + 1;
                return true;
            }
        }

        //
        // Counts the number of consecutive backslashes
        //
        // Preconditions:
        // - Index should be within the bounds of the data string.
        // - The initial data[index] must be a backslash.
        //
        // Return value: The number of consecutive backslashes, including the initial one at data[index].
        private static int CountBackslashes(string data, int index)
        {
            Debug.Assert(index >= 0 && data[index] == MailBnfHelper.Backslash, $"index was not a backslash: {index}");

            // Find all the backslashes. It's possible that there are multiple escaped/quoted backslashes.
            int backslashCount = 0;
            do
            {
                backslashCount++;
                index--;
            } while (index >= 0 && data[index] == MailBnfHelper.Backslash);

            // At this point data[index] should not be a backslash
            Debug.Assert(index < 0 || data[index] != MailBnfHelper.Backslash, $"index was a backslash: {index}");

            return backslashCount;
        }
    }
}
